Bone is the major reservoir of body lead (Pb) stores in humans; and it is now recognized, based upon in vivo and in vitro data, that one skeletal subcompartment of bone Pb is readily exchangeable and modulated in a manner similar to that of calcium (Ca). Moreover, the skeleton is the site of Pb chelation by CaNa2EDTA; and recent clinical observations in children indicate that the skeleton is a target tissue for Pb's toxic effects. Within this context, our laboratory's primary efforts are directed at testing the postulate that pertubations in cellular Ca homeostasis, produced by Pb at relatively low concentrations, is an early and discrete expression of Pb toxicity at the cellular level. Such pertubations are likely to be far reaching and may place the regulation of multiple cellular processes out of the physiological range of normal control through changes in intracellular ionic Ca concentration. Experiments carried out by us in primary monolayer cultures of separated osteoclastic (OC) and osteoblastic (OB) bone cells, during the previous grant period, further confirm and extend this postulate to the point where it can now be verified directly. The specific aims of the proposal are to: 1) characterize the steady state kinetic distribution and modulation of Pb and Ca in OB by desaturation techniques; 2) measure directly by 19F NMR the concentration of cytosolic free Ca and Pb concurrently in OC and OB with and without Pb (and calciotropic hormones) in the medium; 3) determine the specific mechanisms of Pb-Ca interactions at the level of membrane transport, where Pb effects on Ca transport in isolated membranes will be examined in resting and hormonally stimulated OC and OB; 4) define the effects of Pb on Ca-mediated cell functions and the generation and degradation of cAMP; 5) characterize further Pb influences on the integrated functions of OC and OB separately and together in co-culture experiments. Taken together, the proposed quantitative, mechanistic and functional studies in OB and OC have the real potential to define, for the first time, the molecular basis of early Pb toxicity at the cellular level.